Many coherent optical communication systems which use a multi-level modulation signal in order to increase transmission capacity in an optical communication system are disclosed. In these systems, a polarization diversity reception system in which a receiving sensitivity does not depend upon a polarization state is disclosed, like Non-patent literature (NPL) 1. In the system, a polarization beam splitter separates the multi-level modulation optical signal into two polarization optical signals which are mutually orthogonal. A 90-degree hybrid mixes each of separated optical signals with a local oscillation light, and outputs optical signals each corresponding to an in-phase component and an orthogonal component. A photo diode converts the optical signal outputted from each of the 90-degree hybrids into an electric signal.
In NPL 1, correction for change of the receiving sensitivity due to a polarization state of input signals is achieved by digital signal processing. Namely, in NPL 1, an arrangement is not performed in which a polarization plane of the optical signal is conformed to a base line of the polarization beam splitter.
In NPL 1, a Maximal-Ratio-Combining (MRC) method is employed as the digital signal processing correcting the change of the receiving sensitivity due to the polarization state of input signals. In NPL 1, specifically, a power ratio α and a phase difference δ of output signals Ex and Ey of the 90-degree hybrids are calculated. An original optical modulation signal Es is reproduced by the MRC as shown in the equation (1) below,ES=√{square root over (α)}e−jδEx+√{square root over (1−α)}Ey  (1)where j is an imaginary unit. The √{square root over (α)} in the first term of the equation (1) and the √{square root over (1−α)} in the second term thereof are correction terms for maximizing output power.
The e−jδ in the first term of the equation (1) represents a term for correcting the phase difference between Ex and Ey.